The four percentage questions people actually search for, each with the working shown.
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Calculation
Result
30
15% of 200
15 ÷ 100 × 200 = 30
Percentage change is not symmetric. Adding 20% to 100 gives 120, and taking 20% off 120 gives 96, because the second calculation works from a larger starting figure.
Nearly every percentage question is one of four shapes, and mixing them up is the usual source of a wrong answer.
This is the mistake that shows up in news articles constantly. If a rate rises from 4% to 6%, that is a rise of two percentage points, and also a rise of fifty percent. Both are true and they sound wildly different.
When someone quotes a percentage change of a figure that is itself a percentage, check which one they mean before you react to it.
Take 100, add 20 percent, then take 20 percent off. You get 96, not 100. The increase is calculated on 100 and the decrease on 120, so the second one is larger in absolute terms.
This is why a stock that drops 50 percent needs to gain 100 percent to break even, and it is the single most useful thing on this page for anyone reading discount or investment numbers.
Four tabs, one for each shape of question. Choosing the right one is most of the work.
The result updates as you type. No submit button.
The formula used is shown under each result, so you can confirm it answered the question you meant.
How long your streak is, what rate you are running at, and how much room you have left.
Total days, working days, and the same span expressed in weeks and months.
Exact age from a date of birth, plus total days lived and the countdown to your next birthday.
A calculator tells you where you are once. If the thing you actually want is to keep something up, that takes a record rather than a result.
